Generally, as shown in FIG. 5, in a developing section of an image forming apparatus according to an electrophotographic method, such as a copying machine and a printer, are provided an image forming medium 11, such as a photosensitive drum retaining an electrostatic latent image, a developing roller 12 which touches the image forming medium 11 to adhere a toner 20 carried on the surface thereto to convert the electrostatic latent image to a visible image, and a toner transfer roller 13′ (including a toner supplying roller and a cleaning roller for scraping off unnecessary toner) for supplying the toner to the developing roller 12, so that the toner 20 is transferred according to a continuous process from a toner storing section 14 through the toner transfer roller 13′ and the developing roller 12 to the image forming medium 11 to form an image. In FIG. 5, the reference number 15 denotes a transcribing roller, the reference number 16 a charging section, the reference number 17 an exposing section, and the reference number 18 a blade for scraping the toner.
The toner transfer roller is so constituted that an electroconductive elastic material is supported on the circumference of a shaft by means of an adhesive layer in between, in order to protect the developing roller against damages by contact, and to ensure gripping by increasing the contact areas between the rollers. Conventionally, an adhesive with a relatively low melting point as low as 110° C. or less has been utilized for bonding the shaft and the electroconductive elastic material, in order not to damage the electroconductive elastic material by the heat required for bonding.
The mechanism for transferring the toner 20 from the toner transfer roller 13 to the developing roller 12 in the developing system is described in FIG. 6. With respect to the positively (or negatively) charged toner 20, the surface of the toner transfer roller 13 is charged negatively (or positively) by friction with the toner 20 and the developing roller 12. The amount of the charge on the roller surface of the charged toner transfer roller 13 can be controlled by releasing the charge through a shaft 1 containing a metal core. In this case, if the amount of the charge on the surface of the toner transfer roller is limited, the toner sticks to the roller surface less intensively, and, as a result, the amount of the transferred toner can be increased. The term toner transfer roller as used in this Application includes a toner supplying roller and a cleaning roller for scraping off unnecessary toner. That is, it refers to a toner supply and cleaning roller.
Conventionally, the electroconductivity of a toner transferring roller used in a toner cartridge for a laser beam printer (LBP), etc. is required to be set about 103 to 108Ω in terms of the roller resistance at an applied voltage of 100 V in order to control the toner electrostatically as described above. For this purpose, the electroconductive property of the roller has been controlled conventionally mainly by adding an electroconductive filler or an antistatic agent into the electroconductive elastic material. Furthermore, for example, Patent Document 1 discloses a technology, in which a roller is constituted by placing an inner electroconductive layer and an electroconductive elastic layer successively around the circumference of a shaft, and by making the electric resistance in the thickness direction of the inner electroconductive layer higher, for the purpose of providing an electroconductive roller whose electroconductive property can be easily controlled, enabling easy production of a roller with low electroconductivity.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 4-256985 (Claims, etc.)